Upconversion Luminescence Regulation And Temperature Measurement Of Er³⁺/Yb³⁺ Doped Materials

Upconversion luminescence has been widely studied for its physical characteristics and broad application prospects in many domains since it was found.UCL materials can be used in temperature detection,laser anti-counterfeiting,biological imaging,solar cell and so on.The upconversion luminescence properties of rare earth depend on the composition of host materials,crystal structure,temperature,excitation conditions and other factors.Therefore,it's the basis of application to study the regulation law of the above factors on the upconversion luminescence properties.As is well known,Er³⁺/Yb³⁺combination is recognized as an excellent upconversion luminescence system.In this paper,we focused on the Er³⁺/Yb³⁺system,the crystal phase regulation and temperature regulation of upconversion luminescence properties of monoclinic zirconia,cubic zirconia and tetragonal zirconia doped with Er³⁺/Yb³⁺have been studied.The excitation density regulation of Yb F₃:Er³⁺upconversion luminescence color was discussed.The acquired research results are as follows:1.High doped ZrO₂:Er³⁺/Yb³⁺upconversion luminescence phosphors have been obtained by traditional solid-state reaction method,which stabilized the crystal phase by adding Nb⁵⁺.The cubic,tetragonal and monoclinic phases of zirconia were regulated and acquired their pure phase samples.When the total concentration of rare earth ions is?11%,the concentration ranges of cubic,tetragonal and monoclinic zirconia materials are given.The concentration of the three crystal phases was optimized respectively by changing the doping concentration of Yb³⁺and Nb⁵⁺.The optimum concentrations of each crystal phase are c-ZrO₂:2%Er³⁺,14%Yb³⁺,t-ZrO₂:2%Er³⁺,14%Yb³⁺,16%Nb⁵⁺and m-ZrO₂:2%Er³⁺,9%Yb³⁺,12%Nb⁵⁺,respectively.2.High concentration codoped c-ZrO₂:2%Er³⁺,14%Yb³⁺and m-ZrO₂:2%Er³⁺,9%Yb³⁺,12%Nb⁵⁺were successfully prepared by solid-state reaction method with Nb⁵⁺as charge compensation.The green emission near 550 nm come from Er³⁺²H_11/2,⁴S_3/2to ⁴I_15/2and the red emission near 670 nm from Er³⁺⁴F_9/2 to ⁴I_15/2.The UC integral strength of m-ZrO₂ is 4 times that of c-ZrO₂.The comparative study shows that m-ZrO₂has larger absorption cross section and faster radiation transition rate of Er³⁺and Yb³⁺than c-ZrO₂.The larger absorption cross section of Yb³⁺in m-ZrO₂reduces the optimal doping concentration of Yb³⁺,thus inhibiting the reverse energy transfer from Er³⁺green emission level to Yb³⁺as well as the concentration quenching of Yb³⁺.Therefore,m-ZrO₂ shows stronger green and red upconversion emission,while c-ZrO₂ mainly emits red fluorescence.The optical thermal sensing features of thermally coupled green and red levels were detected and analysed.m-ZrO₂:2%Er³⁺,9%Yb³⁺have both intense red emission and intense green emission under 980 nm LD excitation,which shows great potential in temperature sensing based on FIR technology.In our metrical temperature range,the emission intensity of green emission increases monotonically with the increase of temperature from 83?563 K.As for red emission,the obvious feature is that the emission intensity of 654 nm increases with the increase of temperature.As a result,both the green and red levels have the ability of temperature sensing in m-ZrO₂.The maximum sensitivities are0.00414 K^-1and 0.00278 K^-1for green and red respectively.The green-and red-based thermometers are complementary and it should be selected in different temperature ranges.These results show that m-ZrO₂:Er³⁺/Yb³⁺have great application prospects in dual-color optical thermometry.3.Large size cubic zirconia materials were prepared by solid-state method.It was found that both the green and red emission have thermal enhancement in c-ZrO₂:Er³⁺/Yb³⁺.Here was no change of crystal phase in the whole temperature detection range by contrasting luminescence characteristic peak.The variable temperature diffuse reflectance spectrum showed that the absorption didn't change,and the cyclic experiment showed that the thermal enhancement phenomenon was repeated and reversible.The variable temperature near infrared spectra and variable temperature fluorescence decay curves shows that this abnormal phenomenon is closely related to Yb³⁺²F_5/2and Er³⁺⁴I_11/2.According to the literature,we infer that the thermal enhancement of cubic zirconia is related to the capture and re release of electrons by defect state energy levels.4.A fluoride upconversion phosphors Yb F₃:Er³⁺with high concentration and low phonon energy was prepared by high temperature solid state reaction method.It is found that the upconversion luminescence color changes sensitively with the change of 980nm excitation power density,specifically,in the range of 100 m Wcm^-2 low excitation density,the luminescence color changes significantly from green to orange with the increase of excitation density.The research shows that the green emission of Er³⁺is a two-photon process,while the red emission is composed of two-photon and three-photon components,and the three-photon process is very significant at low excitation density.As a result,the red intensity increases with the increase of excitation density faster than green,so the luminescence color changes significantly from green to orange.The three-photon component is not only affected by the excitation density,but also depends on the doping concentration of Er³⁺.With the increase of excitation density,the three-photon component increases faster than the two-photon component.The three-photon process of upconverted red emission in samples with different Er³⁺concentrations was studied.The critical excitation density of the three-photon process for 1%Er³⁺samples is 3.9 m Wcm^-2,and 17.4 m Wcm^-2for 5%Er³⁺samples,all of which are the lowest critical excitation density reported in the world.The correctness of two-photon and three-photon excitation paths is verified by dual color excitation experiments.The significant three-photon process and its low critical excitation density are due to the non-concentration quenching of Yb³⁺in Yb F₃:Er³⁺.This experiment shows that Yb F₃:Er³⁺has potential application in fluorescence anti-counterfeiting technology.